The above formula sketches what happens when a cosmic ray hits a regular atom of Nitrogen (N, on the left). To start with, a natural Nitrogen atom has 7 protons and 7 neutrons, giving it an atom weight of 14. When a cosmic ray hits the Nitrogen atom, it knocks off a neutron, creating from the Nitrogen atom an unstable, radioactive isotope of Carbon, which meanwhile also adds a proton. Normally Carbon has an atomic weight of 12, but this isotope now has an atomic number of 14, making it Carbon-14, also written as ¹⁴C.

Being unstable, ¹⁴C wants to return to its normal state, stable ¹²C. Nothing prevents it from doing this, but it takes a long time. Radioactive ¹⁴C has a half-life of about 5730 years, meaning that if you start out with 100 atoms of ¹⁴C, in about 5730 years you'll have 50, the missing 50 having become ordinary ¹²C. Such conversions of ordinary Nitrogen to ¹⁴Carbon and ¹⁴Carbon to ¹²Carbon happen all the time, though relatively rarely. In Nature, one atom of ¹⁴C exists for every 1,000,000,000,000 normal ¹²C atoms.

Therefore, if you're a photosynthesizing plant using regular atmospheric carbon dioxide for the manufacture of sugar, or an organism consuming those photosynthesizing plants, you can't avoid having one in every 1,000,000,000,000 Carbon atoms in your body an unstable, radioactive ¹⁴C atom.

Of course, when something dies, ¹⁴C atoms cease being added to the dead remains.

In other words, if you find a snail embedded in loess, you can estimate fairly closely when it died by the amount of ¹⁴C still in its body. Something that's been dead for 60,000 years or longer has so little ¹⁴C that the dating technique becomes uncertain. However, our loess was deposited during the last ice age, which ended only 11,000-12,000 years ago, so "carbon-dating" is very helpful for us.

Back during the mid 1960s a team of investigators from Millsaps College in Jackson, Mississippi, headed by J.O Snowden, Jr. and Richard R. Priddy, pulled up to a road cut on the U.S. Highway 61 Bypass at Vicksburg, Mississippi -- a road cut through unmistakably classic loess -- scraped away part of the road cut's weathered face, and found scattered all through the loess white, thumbnail-size fossil snails, just like the ones that once caused Bohumil Shimek's heart to flutter in our "Snails were telling us" section.

Eventually the shells were checked for their ¹⁴C content both at the U.S. Department of Agriculture Sedimentation Laboratory in Oxford, Mississippi, and by a company called Isotopes, Inc., of Westwood, New Jersey.

When the reports on estimated dates of snail death came back, they were simply glorious to behold. The snail-shell samples had been taken at five different levels. Shells in the topmost sample were judged as 17,850 years old, give or take 380 years. The middle sample was placed at 19,250 years old, give or take 350 years. The lowest sample registered 25,300 years old, give or take 1,000 years.

In short, the snails tell us that our loess was deposited during a long period between about 25,000 and 18,000 years ago, which we've known all along was toward the end of the last ice age.